Implantable tissue stimulators such as implantable pacemakers conventionally use a battery as a power means for generating tissue stimulation pulses and operating any electronics and telemetry means contained therein. Although typical pacemaker batteries have a relatively long life, they do eventually require replacement. Such replacement requires removal of the pacemaker and its subsequent reimplantation, thereby causing the patient to incur an additional although minimal risk. Consequently, there has long been a need to determine the life remaining in a pacemaker battery so that it can be replaced at an optimum time, early replacement subjecting the patient to unnecessary risk and late replacement subjecting him to a possibility that his pacemaker will fail. Techniques for determining remaining battery life by loading the battery with predetermined loads, measuring the output voltages of the battery as the load is varied, calculating the internal impedance of the battery from the output voltage measurements, and predicting battery life remaining from the calculated internal impedance are well understood. However, these techniques have not heretofore been utilized in implantable pacemakers because of the complexity involved and the relatively long life of batteries utilized therein. The present invention provides a simple and reliable means for monitoring the life remaining in a pacemaker battery, thereby solving a long standing problem of determining when the battery should be replaced.